The present invention relates to an optical system for an optical pickup device, an optical pickup device and an objective lens, and in particular, to an optical system for an optical pickup device, an optical pickup device and an objective lens which can achieve high density recording or reproducing for information.
As is known as CD (compact disc) or DVD (digital versatile disc), an optical disc has been used for storage of digital data such as accumulation of music information and image information or storage of computer data. Further, with the recent advent of an information-oriented society, there are actual circumstances wherein large capacities of the optical discs are strongly demanded.
In this case, an improvement of recording capacity (recording density) per a unit area can be realized by making a spot diameter of a light-converged spot obtained from the optical system for an optical pickup device, in the optical disc. Since this spot diameter is proportional to λ/NA (in which, λ represents a wavelength of a light source and NA represents a numerical aperture on the image side of the objective lens) as is known widely, a short wavelength of the light source used in the optical pickup device and a high numerical aperture of the objective lens arranged to face the optical disc are effective for making a spot diameter.
Among the foregoing, with respect to the short wavelength of the light source, researches of a violet semiconductor laser generating a laser beam with a wavelength of about 400 nm have been advanced, and it seems to be put to practical use in the near future. In the case of the optical pickup device, in this case, laser power for recording is generally greater than that for reproducing, and therefore, when switching from reproducing to recording, there is sometimes caused the so-called mode hopping phenomenon wherein an output change makes a central wavelength to hop instantaneously by several nanometers. Defocus errors caused by this mode hopping phenomenon can be removed by conducting focusing operations for the objective lens. However, if chromatic aberration of the objective lens is not corrected, troubles such as recording failures caused by defocus errors occur for a period of several nanoseconds up to the moment when the objective lens is subjected to focusing operations. Since longitudinal chromatic aberration of an objective lens grows greater as a wavelength of a light source for a light flux that passes through the objective lens becomes shorter, deterioration of wavefront aberration caused by a mode hopping phenomenon tends to grow greater when a wavelength of a light source becomes shorter. From the reasons above, it is necessary to correct longitudinal chromatic aberration of the objective lens on the optical pickup device wherein a violet semiconductor laser is especially used as a light source.
As an element to correct longitudinal chromatic aberration of the objective lens in a simple structure, there is known a diffractive element utilizing diffracting actions. In the optical pickup device employing a violet semiconductor laser as a light source, an optical pickup device equipped with the diffractive element for correcting longitudinal chromatic aberration of the objective lens and an optical system for the optical pickup device are described in the following Patent Documents 1–3.
(Patent Document 1)
TOKKAI No. 2001-256672
(Patent Documen 2)
TOKKAI No. 2001-108894
(Patent Documen 3)
TOKKAI No. 2002-082280
(Problems to be Solved by the Invention)
An optical pickup device described in the aforementioned Patent Document 1 is one wherein longitudinal chromatic aberration of an objective lens is corrected by a diffractive element arranged in a parallel light flux between a violet semiconductor laser light source and an objective lens, an optical system for an optical pickup device described in the aforementioned Patent Document 2 is one wherein a diffractive structure is formed on an optical surface of a collimator lens for converting a divergent light flux emitted from a violet semiconductor laser light source into a parallel light flux to lead it to an objective lens, and thereby, longitudinal aberration of the objective lens is corrected by the actions of the diffractive structure, and an optical pickup device described in the Patent Document 3 is one wherein a diffractive structure is formed on an optical surface of an expander lens arranged in a parallel light flux between a violet semiconductor laser light source and an objective lens, and thereby, longitudinal chromatic aberration of the objective lens is corrected by the actions of the diffractive structure.
In the aforementioned optical pickup device and the optical system for the optical pickup device, when a wavelength of the semiconductor laser is changed by actions of the diffractive structure in the direction to become longer than a design wavelength of the optical system for the optical pickup device, a light flux which has emerged from a chromatic aberration element and advances to the objective lens becomes a converged light flux, while, a light flux becomes a divergent light flux when a wavelength of the semiconductor laser is changed to become shorter than a design wavelength of the optical system for the optical pickup device, and therefore, by utilizing this characteristic, the longitudinal chromatic aberration of an objective lens can be corrected. However, if an angle of divergence of the light flux advancing toward the objective lens is changed by wavelength changes of a semiconductor laser, in the aforesaid manner, spherical aberration is caused because a magnification of the objective lens is changed.
A wavelength difference of about ±10 nm caused by manufacture errors exists between semiconductor laser individuals each being used as a light source in an optical pickup device. When using a semiconductor laser whose wavelength is deviated from a design wavelength of the optical system for an optical pickup device as stated above, in the aforementioned optical pickup device and the optical system for the optical pickup device, initial adjustment for a position of a collimator lens and that for a position of a semiconductor laser are necessary for eliminating spherical aberration that is caused by changes of magnification of the objective lens, which increases manufacturing cost for the optical pickup device.
In particular, the problem stated above is in a tendency that it is actualized by a single lens representing an objective lens having a high numerical aperture which is generally one way to realize cost reduction and downsizing of an optical pickup device. In a single lens, spherical aberration increases in proportion to the fourth power of the numerical aperture. It is therefore necessary to correct spherical chromatic aberration remaining on the objective lens itself in addition to spherical aberration changes caused by magnification changes of the objective lens, in adjustment of a collimator lens position and in the initial adjustment of a semiconductor laser position. Further, to realize a single lens for an objective lens having a high numerical aperture, it is preferable to use a material having a high refractive index, for securing margin for shifting of the optical axis between optical surfaces.
However, the material having a high refractive index is generally of low divergence, and therefore, an amount of longitudinal chromatic aberration to be corrected by a chromatic aberration correcting element is in a tendency to grow greater. Therefore, for correcting longitudinal chromatic aberration of an objective lens that is made of the material with high refractive index, it is necessary that changes in degrees of divergence of the light flux advancing toward the objective lens from a chromatic aberration correcting element caused by changes of wavelength of semiconductor laser are established to be large, which results in that changes in magnification of the objective lens becomes large when using the semiconductor laser whose wavelength is deviated from a design wavelength of the optical system of the optical pickup device. Therefore, an occurrence of spherical aberration caused by changes in magnification of the objective lens is increased, which increases an amount of initial adjustment for a collimator lens position and an amount of initial adjustment for a semiconductor laser position.
For the problems mentioned above, it is possible to eliminate spherical aberration caused by the changes of magnification of the objective lens, by designing a chromatic aberration correcting element so that spherical aberration (hereinafter, spherical aberration in the case where a wavelength of incident light is changed is called spherical chromatic aberration) may be changed when a wavelength of the semiconductor laser is changed.
However, if spherical chromatic aberration remains on the chromatic aberration correcting element, when using a violet semiconductor laser whose wavelength is deviated from a design wavelength of the optical system for the optical pickup device by manufacture errors, deviation of an optical axis is caused by driving for tracking of an objective lens, and coma which is not negligible is caused, resulting in a fear that satisfactory tracking characteristics are not obtained and recording failure or reproducing failure is caused.